This book seeks to improve knowledge and understanding of residual stress in steel welds by providing a detailed distribution of those stresses measured by non-destructive independent techniques, such as neutron and synchrotron diffraction methods. The work has investigated improved measurement and data interpretation procedures relating to these techniques. One investigation related to the design of stress-free reference samples and the procedure by which they should be measured. Another investigation related to the development of consistency checks on the residual stress measurements to ensure that they met boundary requirements. It was found that these factors were critical for obtaining good correlation between residual stresses measured by synchrotron and neutron diffraction. The work adds to the understanding of the nature of residual stress in welded components. The measurements were compared to the estimates published in the current fitness-for- service codes and provide a sounder base by which finite element models can be judged.